1. Field of the Invention
This invention relates generally to mechanical seal assemblies and, more particularly, to secondary mechanical seal assemblies that can seal a gas and also a liquid.
2. Description of the Related Art
A mechanical seal assembly seals a shaft that is rotatable within a housing and includes a pair of opposed seal face elements, one stationary seal face element that does not rotate relative to the housing and one rotating seal face element that rotates with the shaft. One of these seal face elements is free to move axially toward the other one. Such seal assemblies, for example, can be used in pump systems to seal the pump shaft against either a liquid medium or a gaseous medium to substantially prevent the medium from flowing along the shaft and out the housing to the atmosphere. The seal face elements are pressed together at their opposing front surfaces by a closing force, which typically is supplied by one or more axially-oriented springs and the pressure of the sealed medium on the back side of the seal face elements.
In general, mechanical seal assemblies that are used to seal a liquid medium have seal face elements with opposing surfaces that are flat. The opposing surfaces do not actually come in contact with each other because the liquid medium has sufficient pressure to push the seal face elements slightly apart, providing an opening force. The opening force allows a thin film of the liquid medium to act as a lubricating film between the seal surfaces, on the order of a few microns, that prolongs seal life and reduces seal wear. In general, mechanical seal assemblies that are used to seal a gaseous medium have seal face elements with opposing surfaces that are provided with stepped pads or spiral grooves. Because a gaseous medium provides less efficient lubrication than a liquid medium, the stepped pads or spiral grooves are needed to create surface deformities that generate a sufficient opening force to push apart the seal face elements. The seal face elements are thereby kept apart, which prolongs seal life and reduces seal wear.
In critical operations, or applications where a volatile medium is involved, it is often necessary to provide a redundant sealing system that will prevent the medium from flowing to the atmosphere, contain a leak, and meet safety requirements for stable operation of the system. A redundant sealing system includes a primary seal assembly that seals the system medium from flowing along the shaft and a secondary seal assembly that seals the very minute leaks usually produced by the primary seal assembly and that temporarily seals the system in the event of a sudden failure of the primary seal assembly.
If the redundant sealing system is to seal against a liquid medium, the redundant sealing can be provided by a tandem seal design having a primary liquid mechanical seal assembly, a secondary liquid mechanical seal assembly spaced apart from the primary seal assembly, and an auxiliary liquid buffer reservoir and support system between the two seal assemblies. The liquid buffer effectively seals the liquid medium from the secondary seal assembly. In this way, only the liquid buffer is allowed to leak past the secondary seal assembly to the atmosphere. Although tandem liquid seals provide redundant sealing of the media and are reliable, they can be costly because of the additional equipment and maintenance requirements.
The very minute leaks usually produced by a primary liquid-type seal assembly often are of the gaseous phase. This occurs because the liquid medium, in escaping from the primary liquid-type seal assembly, is converted to a gaseous phase. Therefore, the secondary seal assembly can be provided by a gas-type seal assembly. Because the secondary seal assembly also must maintain a seal if the primary seal fails, the secondary seal assembly must be capable of operating as a gas-type seal and transitioning to a liquid-type seal. Unfortunately, the opening forces generated by a conventional gas-type seal assembly with stepped pads or spiral grooves are such that excessive leakage results when a liquid medium is introduced. Moreover, the stepped pads or grooves can become fouled with debris or worn off during dry operation, losing their effectiveness and resulting in excessive wear.
From the discussion above, it should be apparent that there is a need for a relatively simple and inexpensive secondary seal assembly that can seal a gas medium without suffering from excessive wear or fouling and that also can transition to seal a liquid medium without suffering from excessive leakage. The present invention satisfies this need.